This invention relates to electrochemical analysis commonly referred to as amperometric analysis or alternatively as polarographic analysis. Specifically, this invention relates to an electrode assembly designed to perform as a sensor for monitoring the concentration of selected components in blood and is particularly applicable to such a sensor when used for the in vivo monitoring of blood gas concentrations, e.g., oxygen tension in blood vessels of living animals including humans.
In U.S. Pat. No. 3,912,614 issued on Oct. 14, 1975 to S. B. Spracklen and H. Watanabe Capistrano, a sensing electrode assembly for incorporation into a sensor has been described. Briefly, the electrode assembly comprises a fine gauge electrode composed of a noble metal which is covered by a selectively permeable membrane. The sensing electrode is immersed into the stream to be tested, e.g., into the blood vessel of a living animal and is polarized. In the case of measuring oxygen tension in blood, for example, oxygen in the blood tends to diffuse through the membrane to the electrode where the oxygen is reduced by reacting with water in the blood and accepting electrons from the electrode to form hydroxyl ions. Where the electrode is made part of a closed electrical circuit including, for example, a reference electrode, connecting electrical wiring, means for imposing a potential at the sensing electrode and means for detecting current flow, the flow of the electrons from the sensing electrode constitute a current flow through the electrical circuit and the amperage of that current flow is directly related to the oxygen tension in the blood stream.
As has been described in the aforementioned U.S. Pat. No. 3,912,614 as well as in U.S. Pat. No. 3,757,771 issued to F. A. Ruegg, et al. on Sept. 11, 1973; such a device has been employed in monitoring changes in blood oxygen in living animals, including humans. In particular, it is useful for the continuous monitoring of such blood oxygen tension in the case where conventional blood analysis is impractical. Such a circumstance exists in the case of new born infants and especially, for prematurely born infants where, because of the relatively small quantity of total blood present in the body of such infants, it is impractical and dangerous to frequently extract blood samples for in vitro testing using conventional methods. Instead, the use of a continuous in vivo monitoring system, as is described herein, is a far more preferable alternative.
Ideally, the sensor electrode is inserted into the blood vessel of the living animal and by use of an amperage sensing meter, calibrated to read directly in partial pressure of oxygen, the oxygen blood level in that vessel can be immediately and continuously monitored.
Unfortunately, the sensing devices heretofore utilized have fallen short of this ideal situation. It has been experienced that when the sensing electrode is first introduced into a blood vessel, the initial reading is relatively high. Thereafter, notwithstanding the fact that the blood oxygen tension remains constant, the reading begins to decrease, first at a rapid rate and then at a decreasing rate. It is only after the passage of a relatively long time period, e.g., six hours, that the rate of decrease becomes relatively low enough so as to provide a reliable base line for measuring significant changes in blood oxygen tension.
This time period for allowing the sensor to reach a stable, reliable state of operation is intolerably long in many of the potential uses for the sensor where the first few hours of monitoring are critical to the patient's welfare. Accordingly, there is a need for decreasing this time period.